World Famous in Palau!
An earthcache at a world famous* natural arch amongst the beautiful Rock Islands of Palau, by funkymunkyzone and geocoinsnz.
You will need a Rock Island Use or Jellyfish Lake permit from the Koror State Government in order to visit the natural arch and complete this earthcache, or indeed visit anywhere else in the Rock Islands. The Jellyfish Lake permit allows for accessing the Rock Islands as well as Jellyfish Lake.
If you visit here with any of the tour operators, they will organise the permit for you and you will pay them directly. As of time of publication of this earthcache, the Rock Islands Use permit costs 50USD and the Jellyfish Lake permit costs 100USD. These permits may also be purchased from the Koror State Ranger's Office or the Koror State Treasury. If you are not going with a tour operator please make sure you are familiar with the map of locations where landing is allowed as landing is prohibited to non-residents through most of the Rock Islands.
*in Palau
Geology of Palau
The archipelago of approximately 340 islands that make up Palau sits on the edge of the Philippine Sea tectonic plate. Here, the massive Pacific plate subducts underneath the smaller Philippine Sea plate creating upthrust on the eastern edge of the Philippine Sea plate, and a deep trench at the edge of the Pacific plate.
The islands sit on the Kyushu-Palau Ridge, which is a 2600km long seabed ridge that extends southeast from the Japanese island of Kyushu, turning south towards Palau and finally southwest before intersecting with West Papua, Indonesia. Along this line there exists a chain of long extinct volcanoes.
Palau itself is the result of a massive volcanic eruption creating a 150km long volcanic ridge, that is now slowly subsiding. This created the perfect environment for free floating coral larvae to then settle onto this substrate and begin to build a new reef. After millions of years of deposition the tiny coral brick layers had formed a massive limestone cap on top of the sinking volcanic ridge.
A further episode of violent subduction of the Pacific plate caused massive upthrust of the Philippine Sea plate approximately 35 million years ago and forced upwards the limestone cap exposing it above sea level. Today’s world famous “Rock Islands” are the heavily eroded remnants of a once vast exposed limestone reef. The islands of Babeldaob and Koror are all that remains of the still subsiding volcano.
Geology of the Rock Islands
Palau's Rock Islands are the eroded remnants of a vast coral reef, built over a long extinct volcanic ridge, that has been thrusted upwards by tectonic plate movement. They have been exposed to various forms of erosion above sea level for approximately 35 million years.
Some of the forms of erosion include:
Wave Action - Most noticeable about the rock islands is the indentation at sea level, giving the islands a mushroom type of shape. This is largely, but not exclusively, due to wave action eroding the foundation of the islands. Wave action pounding on the rock, along with hydraulic action as incompressible water is pushed into cracks and fissures in the rock.
Wind Action - wind, potentially carrying sand or other abrasive particles, erodes the islands.
Chemical Erosion - Decaying organic matter along with rainfall creates a weak acidic solution that dissolves the limestone rock as it seeps down through the rock, or flows down the surface of the islands. Many of the islands exhibit stalactite-like features in the ceiling of the undercut areas where minerals have precipitated out of the solution as it drops into the sea.
Seismic Activity - Not exactly a form of erosion, but a means by which the islands can crumble if the resultant shaking exploits rock that has been weakened already by other forms of erosion.
Biological Activity - Life forms live on the rock, particularly within the intertidal area, and under the surface, which damage the rock either while feeding on algae, or by directly burrowing into the rock.
This Earthcache
When you visit this area, you will undoubtedly see a number of natural bridge formations that have resulted from erosion of irregular shaped islands such that in the course of time, the erosional indentation at sea level has broken through to the other side of the island. As time goes on, these will form larger arches, which will eventually collapse and become two islands.
The natural arch that is the subject of this earthcache is a popular and significant example of a sea arch that has formed in this way. To log a find on this earthcache, you will need to observe the natural arch at the published coordinates and complete the following simple tasks. Answers to these questions must be sent to the cache owner (by clicking on the profile link at the top of the page) - you should do this before logging your find online:
1. Estimate the height and width of the arch, in metres.
2. Do you think the formation of this arch is largely a slow gradual erosion process, or do you think it has formed largely from sudden events? Explain why you believe this to be the case (from your observations at the coordinates).
3. Take a photo of yourself (or your GPS or some other personal item) while visiting the Rock Islands. Post this photo with your find log - don't send it to the cache owner.
You can log a find on this earthcache as soon as you have sent to the cache owner your answers to the above questions. Such logs that do not correspond with answers, or do not have a photo, will be deleted.